Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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478
FXUS63 KGRR 211928
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
325 PM EDT Thu Sep 21 2017

LATEST UPDATE...
Synopsis/Short Term/Long Term/Hydro/Marine

.SYNOPSIS...
Issued at 325 PM EDT Thu Sep 21 2017

Hot and humid weather will continue through the rest of the week and
weekend. Isolated thunderstorms will continue to develop near to
east of a line from Alma to Kalamazoo late this afternoon into early
evening but most locations will not receive any beneficial rainfall.
Dry weather is forecast Friday through the weekend into early next
week and the next good chance for rain will not come until the
middle of next week.

&&

.SHORT TERM...(This evening through Saturday night)
Issued at 325 PM EDT Thu Sep 21 2017

Strong instability has developed this afternoon near to east of a
line from Alma to Kalamazoo where ml cape values have reached 2500
j/kg. Isolated convection continues to develop attm in this area
along and just ahead of a weakening front.

Given vis sat/radar trends attm and building instability we will
feature a chc of a shower or storm in the fcst near to east of a
line from Alma to KAZO from mid to late aftn into early eve. I did
add patchy fog to the fcst during the early morning hours Friday
given the very humid airmass in place and very little boundary layer
mixing at that time.

Hot and humid and dry wx will continue Friday through Saturday
wiht sfc/upper ridging in place and persistent south to southwest
flow. High temps both days will reach the upper 80`s to lower 90`s
with low temps ranging from the middle 60`s to lower 70`s.

.LONG TERM...(Sunday through Thursday)
Issued at 325 PM EDT Thu Sep 21 2017

Big changes to the upper air pattern occur out in the long term
portion of the forecast. The upper ridge over the Great Lakes, which
will have been in place for days by Sunday breaks down next week.
The ridge which is responsible for our hot weather is essentially
squeezed by an advancing deep trough out of the west and Hurricane
Maria. 500mb heights of 590dm at 12z Sunday drop into the 560s DM`s
on Thursday of next week.

We remain well above normal temps on Sunday and Monday with highs
well into the 80s. Dry weather is forecast both days with surface
ridging slowly sliding to the east. On Tuesday the approaching front
will still be upstream. Temperatures will remain well above normal,
but may be tempered a bit by clouds and slightly cooler 850mb temps.

There are some timing differences, with the ECMWF being a bit
quicker, but the front that changes our air mass (and ushers us into
fall like air) moves through on Wednesday. Showers and storms are
possible with the frontal passage. Much cooler air moves in behind
the front with highs in the 70s on Wednesday and 60s on Thursday.
The air is cool enough on Thursday in fact that we would be looking
at lake effect rain showers.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 205 PM EDT Thu Sep 21 2017

Limited aviation concerns the next 24 hours with mainly VFR
weather forecast. The exception to the rule will be light fog
late tonight. Between mainly 09z and 14z we will see fog form.
3-5SM visibilities are expected in spots tonight. Localized fog
down into the IFR category is possible, especially towards KLAN
and KJXN.

Otherwise, scattered to broken clouds with bases at or above
15,000ft are expected. Some scattered to possibly broken diurnal
cumulus development is also expected both today and on Friday.
Bases on the cumulus deck should be around 4,000ft.


&&

.MARINE...
Issued at 325 PM EDT Thu Sep 21 2017

Fairly minimal wind speeds and wave heights are forecast through
Friday with south to southwest winds of mainly around 5 to 10 kts
and wave heights of less than two feet. Patchy fog is possible
mainly early Friday morning.

&&

.HYDROLOGY...
Issued at 325 PM EDT Thu Sep 21 2017

A few thunderstorms may develop during the afternoon and early
evening to the east of US 131. These storms will have the potential
to produce very heavy rainfall. Coverage of these storms will be low
if development occurs, and most locations will remain dry. Dry
conditions with near-record warmth is then expected Friday through
the weekend. The next chance for rain is late Tuesday night and
Wednesday.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

SYNOPSIS...Laurens
SHORT TERM...Laurens
LONG TERM...Duke
AVIATION...Duke
HYDROLOGY...HLO
MARINE...Laurens



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